Referring to FIG. 1, a nail gun of the prior art comprises an electrical or gas driver 10 and a nail feeder 20 mounted in the front of the driver 10. The nail feeder 20 further comprises a guide mount 21, a slide head 22, a compressional spring 23, a nail-belt transmission mechanism 24 and an adjustment bracket 25.
As shown in FIG. 3, the guide mount 21 further comprises a left half 21A and a right half 21B, whereby they can sandwich the front end of the driver 10 when they are coupled. The inner wall of the left half 21A is provided with a guide slot 211 having a horizontal section and a sloppy section.
As shown in FIG. 3, the slide head 22 is slidably housed within the guide mount 21, which consists of a left half 22A and a right half 22B. The outer wall of the left half 22A is provided with a guide slot 221 taking the shape of an arc.
As shown in FIG. 3, the compressional spring 23 is mounted between the slide head 22 and the guide mount 21 for exerting an outward pushing force on the slide head 22.
As shown in FIG. 2, the nail-belt transmission mechanism 24 is housed within the slide head 22, which comprises a transmission gear wheel 241, a ratchet wheel 247 and a retaining part 248. The transmission gear wheel 241 has an axial hole 242 provided with an unidirectional control unit 243. The unidirectional control unit 243 is a unidirectional bearing set and coupled with a transmission axle 244 going through the axial hole 242, whereby the transmission axle 244 can rotate only in the clockwise direction. The outer end of the transmission axle 244 is provided with a link 245 whose other end is provided with an axle 246 that bulges out of the guide slot 221 on the left half 22A. The axle 246 is further extended into the guide slot 211 on the left half 21A when the guide mount 21 and the slide head 22 are coupled. Moreover, the ratchet wheel 247 is mounted on the inner wall of the right half 22B of the slide head 22. The retaining part 248, being a spring with an arm, is mounted on the same wall for assuring an equal-pace and unidirectional rotation of the ratchet wheel 247 and the transmission axle 244 only in the counter-clockwise direction.
As shown in FIG. 1, the adjustment bracket 25 is mounted in the front of the slide head 22 for adjusting the spacing between the adjustment bracket 25 and the slide head 22 for accommodating nails of different lengths.
As shown in FIG. 4, to use the nail gun, the slide head 22 is pushed against a wall and therefore retract into the guide mount 21, whereby the axle 246 in the nail-belt transmission mechanism 24 slides along the guide slots 211 and 221. The link connected to the transmission axle 244 rotates within the unidirectional bearing set in the clockwise sense. At the same time, the retaining spring 248 is tilted against the ratchet wheel 247, and therefore the transmission gear wheel 241 and the ratchet wheel 247 will not rotate accordingly. Whereby, the driver 10 will lock on a screw nail. When the slide head 22 is released, the compressional spring 23 will slide along the guide mount 21, driving a backward slide of the axle 246 along the guide slots 211 and 221 and then causing the transmission axle 244 to rotate in the counter-clockwise sense. However, since the transmission axle 244 rotate in the clockwise sense within the bearing set, the transmission axle 244 will drive the transmission gear wheel 241 and the ratchet wheel 247 instead. Further, because of the engagement between the retaining spring 248 and the ratchet wheel 247, the counter-clockwise rotation is step by step with a fixed spacing. As shown in FIG. 3, the nail belt 30 can feed nails by the counter-clockwise jump of the transmission gear wheel. The adjustment bracket 25 is mounted in the front of the slide head 22 for adjusting the spacing between the adjustment bracket 25 and the slide head 22 for accommodating nails of different lengths.
However, the nail of the prior art has the following disadvantages.                (1) Since the nail-belt transmission mechanism 24 is realized by a counter-clockwise, equal-spacing jumped rotation of the transmission gear wheel 241 driven by a unidirectional bearing set, the slide head 22 has to retract into the guide mount 21 for a fixed distance. Uneven pushing of the slide head 22 may lead to uneven transmission of the nail belt, causing jamming of nail gun.        (2) As shown in FIG. 5, the retaining spring 248 within the nail-belt transmission mechanism 24 may run into material fatigue after repetitious uses.        